1. Field of the Invention
The present invention relates to an antenna device preferably for use in short-range wireless communication and a wireless communication apparatus including the antenna device.
2. Description of the Related Art
NFC (near field communication), which uses a 13 MHz frequency band, is a short-range wireless communication standard. NFC is expected to be used for electronic apparatuses, such as cell phones. If NFC were widely used, wireless communication between electronic apparatuses could be performed by simply bringing the electronic apparatuses close to each other. As a result, data transfer and data communication could be easily performed. Therefore, various applications of NFC, such as contactless cashless payment, are being developed. In the present specification, electronic apparatuses that are capable of performing short-range wireless communication will be referred to as wireless communication apparatuses.
Japanese Unexamined Patent Application Publication No. 2004-56413 describes an existing wireless communication apparatus of this type. Japanese Unexamined Patent Application Publication No. 2004-56413 describes a portable apparatus having an IC card function, which is a wireless communication apparatus. The portable apparatus includes a portable apparatus module that is removably mounted therein. The portable apparatus module includes an integrated control circuit, a memory, and an antenna coil. The portable apparatus further includes a second antenna coil that is electromagnetically coupled to a first antenna coil, which is included in the portable apparatus module. The portable apparatus performs contactless communication with an external reader/writer through the second antenna coil.
Japanese Unexamined Patent Application Publication No. 2002-175508 describes another wireless communication apparatus. Japanese Unexamined Patent Application Publication No. 2002-175508 describes a contactless data carrier apparatus, which is a wireless communication apparatus. The data carrier apparatus includes an antenna coil disposed on a semiconductor chip and a booster coil disposed adjacent to the antenna coil. The booster coil is larger than the antenna coil. By using such a booster antenna for wireless communication, the peak of resonance of a signal received from an external reader/writer can be made higher. Therefore, the communication range can be extended as compared with a case where the antenna coil on the semiconductor chip is used.
In recent years, reduction in size and increase in packing density of electronic apparatuses have been rapidly progressing. Therefore, it is difficult to find enough space in which to install an antenna coil when mounting a communication module or the like compliant with an NFC standard in a casing of an electronic apparatus. As a result, the antenna coil is disposed too close to surrounding components in the electronic apparatus, and unwanted coupling between the antenna coil and surrounding components may occur. Such unwanted coupling causes a problem of a decrease in the performance of the antenna coil.
To perform short-range wireless communication, hot spots of wireless communication apparatuses are brought close to each other. The term “hot spot” refers to a portion of a surface of a casing of a wireless communication apparatus at which the magnetic field of an antenna coil is strong. To date, wireless communication apparatuses have been designed so that the hot spot is located above the antenna coil. However, as described above, it has become difficult to provide enough space in which to install the antenna coil. Therefore, there is a problem in that it is difficult to form a hot spot at a desired position.